I/O (Input/Output) Selection – Senland

In the 90s of the 20th century, PLC has formed a variety of PLCs: micro, small, medium, large and giant. According to the number of I/O points, it can be divided into five types: micro PLC (32 I/O), small PLC (256 I/O), medium PLC (1024 I/O), large PLC (4.69 I/O), and giant PLC (8195 I/O).

The communication between the PLC and the industrial production process is completed by means of I/O interface modules. PLC has many I/O interface modules, including switching input module, switching output module, analog input module, analog output module and some other special modules, which should be selected according to their characteristics.

I/O (Input/Output) Selection – Senland

(1) Determine the number of I/O (input/output) points

When determining the number of I/O points required according to the requirements of the control system, a reserve of 10%-20% should be added to add the control function at any time. For a control target, the number of I/O points should be different depending on the control method chosen or the level of programming. Table 2 lists the number of switching I/O points required for typical transmission equipment and commonly used electrical components.

(2) Switching I/O (input/output)

The switching I/O (input/output) interface accepts signals from sensors and switches (e.g. pushbuttons, limit switches, etc.) as well as control devices (e.g. indicators, alarms, motor starters, etc.). Typical AC input/output signals are 24 – 240 V and DC input/output signals are 5 – 240 V. Although the input circuits vary depending on the manufacturer, some characteristics are the same, such as the flutter circuit used to eliminate faulty signals. In addition, most input circuits have an optional isolation circuit between the high-voltage power supply input and the operating logic portion of the interface circuit. When evaluating discrete outputs, fuses, transient surge protection, and isolation between the power supply and logic circuitry should be considered. The fuse circuit may be more expensive at the beginning, but it may be less expensive than the fuse in an external device.

(3) Analog I/O (input/output)

The analog input/output interface is typically used to sense the signals generated by the sensor. These interfaces can be used to measure flow, temperature, and pressure, and can be used to manipulate voltage or current output devices. Typical ranges for these interfaces are -10 to +10 V, 0 to +11 V, 4 to 20 mA, or 10 to 50 mA. Some manufacturers plan special analog interfaces on the PLC, so that low-level signals such as RTDs, thermocouples, etc. can be accepted. In general, these interface modules can be used to accept different types of thermocouple or RTD mixed signals on the same module.

(4) Special function I/O (input/output)

(Not visible yet) When selecting a PLC, users may face some special types of I/O limitations that cannot be accomplished with standard I/O (e.g., positioning, fast input, frequency, etc.). In this case, the user should consider whether the supplier offers a module that specifically contributes to the maximum reduction of the control effect. Some special interface modules can process a portion of the field data themselves, freeing the CPU from heavy mission processing.

(5) Intelligent I/O (input/output)

At present, PLC manufacturers have successively launched some intelligent I/O modules. Generally, the intelligent I/O module has its own processor, which can pre-specify the processing of input or output signals, and send the processing results to the CPU or direct output, which can improve the processing speed of the PLC and save the memory capacity.

I/O (Input/Output) Selection – Senland

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